sealed class Tree {
    
    data class Leaf(val value: String) : Tree()
    data class Node(val children: List<Tree>) : Tree()
}

when

fun stringRepresentation(tree: Tree): String {
    return when (tree) {
        is Tree.Leaf -> tree.value
        is Tree.Node -> tree.children.joinToString(separator = " ", prefix = "[", postfix = "]") { stringRepresentation(it) }
    }
}